Allow unlimited character pushback in unget-char.

Fixing read_until_match will require this feature.

* socket.c (dgram_get_char): Treat unget_c as a
cons-based stack; pop a character from it if
available.
(dgram_unget_char): Push the character onto unget_c
rather than storing the characer into unget_c.

* stream.c (stdio_get_char, stdio_unget_char): Closely
analogous changes to the ones in dgram_get_char
and dgram_unget_char, respectively.

* txr.1: Documentation improved and updated.

1 files changed, 4 insertions, 11 deletions

diff --git a/socket.c b/socket.c
index 7cb119ae..102127b9 100644
--- a/socket.c
+++ b/socket.c
@@ -434,10 +434,9 @@ static int dgram_get_byte_callback(mem_t *ctx)
 static val dgram_get_char(val stream)
 {
   struct dgram_stream *d = coerce(struct dgram_stream *, stream->co.handle);
-  val uc = d->unget_c;
-  if (uc) {
-    d->unget_c = nil;
-    return uc;
+
+  if (d->unget_c) {
+    return pop(&d->unget_c);
   } else {
     wint_t ch = utf8_decode(&d->ud, dgram_get_byte_callback,
                             coerce(mem_t *, d));
@@ -455,13 +454,7 @@ static val dgram_get_byte(val stream)
 static val dgram_unget_char(val stream, val ch)
 {
   struct dgram_stream *d = coerce(struct dgram_stream *, stream->co.handle);
-
-  if (d->unget_c){
-    d->err = EOVERFLOW;
-    uw_throwf(file_error_s, lit("unget-char overflow on ~a: "), stream, nao);
-  }
-
-  d->unget_c = ch;
+  mpush(ch, mkloc(d->unget_c, stream));
   return ch;
 }